1. Technical Field
This invention relates generally to radial lip seals for use with relatively rotatable members such as wheel axle applications, and in particular to lip seals wherein the contacting lip is formed of two dissimilar materials.
2. Related Prior Art
Lip-type fluid seals typically employ a flexible sealing member that is fixed to an outer metallic case and includes one or more sealing lips along a radially inward edge thereof. The selection of the sealing member material is determined by a number of factors, including the environment in which the seal will be operating. Elastomeric materials, such as rubber, are often employed as the sealing lip material in applications where the lip seal is exposed to a dirty environment. For example, seals used in heavy equipment applications for sealing axle bearings are exposed on the air or atmosphere side of the seal to various contaminants which would be harmful to the bearings, including water, salt, and abrasives such as dust, mud, sand, etc. In other applications, contaminants may be present in lubricant on one side of the seal. In either case, elastomers are generally preferred as the sealing lip material over other materials, such as polytetrafluoroethylene (PTFE) materials, as the elastomers are far more resistant to wear under such contaminated conditions than those of the PTFE materials, which generally perform poorly.
Environmental conditions which favor PTFE materials over the elastomers for the selection of the sealing member include low and high temperature and/or dry or low lubricant running conditions. The PTFE materials are generally more wear resistant than elastomers under such conditions.
Difficulties arise, however, when seals operate in environments that favor neither material. For example, in the heavy equipment application described above, it is not uncommon for the seal to be subjected not only to a contaminated environment, but also to low or high temperature and/or low lubricity conditions. In the past, the elastomers were selected over PTFE materials, as the elastomers are more tolerant of low or high temperature and/or low lubricity conditions than are the PTFE materials of a contaminated running condition. Thus, there is a need in the industry for a lip seal that performs well in a contaminated environment and low or high temperature and/or low lubricity operating conditions.